Method of making a hybrid liquid-solid propellant system with encapsulated oxidizingagent and metallic fuel



United States Patent No Drawing. Filed Mar. 26, 1959, Ser. No. 802,257

. Claims. (Cl. 149-21) The present invention rel-ates to encapsulatingliquids and propellant systems wherein the liquid portion isencapsulated in a polymeric substance. The invention is concerned notonly with the method for encapsulating the liquid, which is thereaftercompounded with solid materials, but also the novel propellant systemobtained.

There is considerable interest in the use of solid propellant systems inrockets because they are less diflicult to handle and the rocket itselfis'simpler to design and construct. The type of rocket to which thepresent invention has particular application is the type of rocketpropulsion device commonly designated as a pure rocket, that is to say,a thrust producer which does not make use of the surrounding atmosphere.The propellant system in these rockets comprise fuel and oxidizingmaterial which react either spontaneously or upon ignition to producegases which escape from the combustion chamber through a thrust nozzleto impart thrust to the rocket. Liquid oxidizing agents are generallymore effective than their solid counterparts because of their highreactivity. However, considerable difficulty has been encountered inattempting to combine liquid oxidizing agents with solid fuels becauseof the tendency of the two materials to prematurely react.

An object of the invention is to provide a new rocket propellant systemwhich utilizes a liquid oxidizing agent and a solid fuel. Another objectof the invention is to provide a method for encapsulating liquidoxidizers so as to render them stable in the presence of fuel untilignited.

In accordance with the present invention a liquid oxidizing agent isencapsulated with a polymeric substance and the capsule is combined withfuel to produce a hybrid propellant system having good stability andhigh thrust. In carrying out the invention, drops of oxidizing agentpass through a zone containing polymer in a liquid form and the dropscoated with a thin film of said polymer are recovered from theencapsulating zone. The encapsulated drops of oxidizing agent may bewashed with a liquid which is eithera solvent or nonsolvent for thepolymeric substance surrounding the drop. The washing removes any excesspolymer and reduces its tackiness. While tackiness is not an undesirableproperty because it assists in bringing the oxidizing agent into closercontact with the fuel, which sticks to the surface of the polymer, itmay be desirable in some instances to have a tack-free capsule. Thecapsule of oxidizing agent, which may vary in diameter from to $4,", isadmixed with a solid fuel, preferably a metal selected from Groups II-A,III-A, IV-A and IV-B of the Periodic Chart of Elements, Langes Handbookof Chemistry, 8th Edition, pages 56-57, or metal hydride in a weightratio of 1 to 2:1. Suitable fuels include aluminum powder, magnesiumpower, boron powder, aluminum hydride, magnesium hydride, titaniumhydride and boron hydrides. The two components may be contacted witheach other in any suitable manner which may be either batchwise orcontinuous. It is often advantageous to have between about 0.5 and 50wt. percent, including the weight of hte encapsulating polymer, of abinder present which will maintain the oxidizing agent and fuel in asemi-rigid form which is somewhat flexible and has a tensile strength ofat least 50 lbs. psi. The binder, which may be a low molecular weightresin, rubber or 3,395,055 Patented July 30, 1968 "ice asphalt, can beblended with the fuel prior to mixing the fuel with the oxidizing agent.Some examples of suitable binders are isoprene-isobutylent butyl rubber,polyisobutylene, polyethylene and polypropylene. In order to obtain veryhigh specific impulses, it is best to use less than about 10 wt. percentof both binder and polymer. The compounded propellant may be molded intoany desired shape prior to placing it in the combustion chamber of therocket.

The oxidizing agent which is a liquid, such as red fuming nitric acid(RFNA), white fuming nitric acid (WFNA) or concentrated nitric acid, maybe sprayed in the form of discrete drops onto the surface of a solutioncontaining from about 10 to Wt. percent polymer dissolved therein. Ifthe oxidizing agent does not react with the polymer to form a filmaround it, it is necessary to add reactive substances to the agent topromote encapsulation. In the case of RFNA this may be a catalyst orpromoter for nitration reactions such as HF, BF acetic anhydride,acetone, sulfur and inorganic salts. It has been noted that the dropsare most advantageously encapsulated with a film of polymer when thesolution contains about 20 to 30 wt. percent of polymer. Since thethickness of the encapsulating substance will vary according to thecontact time and the viscosity of the polymer solution, theencapsulating solution should be adjusted accordingly. In order toobtain capsules having walls of 0.5 to 40 mils thick, the drop should bein contact with the encapsulating solution for 10 seconds to 30 minutes.In order to avoid the presence of an excess amount of polymer in thepropellant system, it is advantageous to control the encapsulatingreaction to produce capsules which are about of an inch in diameterhaving Wall that are 1 to 5 mils thick.

The oxidizing agent is preferably one which reacts with the polymer toform an insoluble fil'm which surrounds the oxidizing agent. However,any oxidizing agent which is compatible with a nitrating substance, e.g.concentrated nitric acid, may be used in admixture with said nitratingsubstance. In one embodiment, relatively small drops of an oxidizingagent containing a minor amount of nitrating agent, i.e. about 1 to 20wt. percent, are contacted with a polymer solution at about P. which isnot too viscous to permit the drops of oxidizing agent to pass throughit. In many instances the nitrating agent is also an excellent oxidizer,e.g. RFNA. Among the oxidizing agents which may be combined with say aninorganic nitrating agent are the nitroparaffins, especially themethane, ethane and propane derivatives, e.g. tetranitromethane,nitromethane, nitroethane and nitropropane. Substituted C to Cnitroparafiins, such as bromotrinitromethane, may also be employed. Thepolymer should contain reactive sites which permit it to combine withthe oxidizing agent-nitrating agent mixture and thus form a semi-rigidfilm around the drop. Suitable polymers include such substances aspolybutadiene, styrenebutadiene rubber, polyisobutylene,isobutylene-isoprene rubber (butyl rubber), polyethylene, polypropylene,styrene-isobutylene polymer and fluorohydrocarbon polymers. The polymersgenerally have a hydrocarbon backbone and preferably have a Staudingermolecular weight of about 10,000 to 150,000. The encapsulating reactionmay be carried out from 30 to 125 F. under atmospheric pressure, and foreconomic reasons it is preferable to employ temperatures of 60 to F. Thesolvent, which is generally organic, should be inert and capable ofdissolwing substantial amounts of polymer. As the polymer reacts withthe oxidizing agent it forms an insoluble film around each drop and thecoated drops are recovered by any suitable manner, such as filtration ordecantation. Suitable solvents for the encapsulating reaction includesuch things as carbon tetrachloride, hexane, heptane, octane,dichloroethane, etc.

Since it may be desirable in some instances to have a strong polymerfilm around the oxidizing agent which is capable of withstandingconventional handling, it is advantageous to include curing andaccelerating agents in the rubber solution so that the polymer filmencapsulating the oxidizing agent may be cured either at roomtemperature or elevated temperatures. For instance, a small amount ofsulfur may be compounded with the polymer before it is dissolved in thesolvent and treated with the oxidizing agent. Of course, curing agentsshould only be used where the polymer is capable of being vulcanized,e.g. a polymer having double bonds or active groups. Additional strengthmay be imparted to the polymer by compounding it with other well knowncompounding substances before dissolving it in the inert organicsolvent. Any excess polymer may be removed from the capsules by washingthem with additional solvent. Known solvents, such as organic alcoholsand ethers, may be used to reduce the tackiness of the capsules.

The following examples are given to more fully illustrate theencapsulating process of the present invention.

Example 1 A 25 wt. percent solution of styrene-isobutylene copolymercontaining 60 wt. percent styrene and having a Staudinger molecularweight of 40,000 was prepared by shaking the polymer in carbontetrachloride at room temperature overnight. The polymer solution wasplaced in a vessel which was 2 ft. high and 1 in. wide and drops of redfuming nitric acid were introduced at the top of the vessel andpermitted to fall through the polymer solution to the bottom of thevessel where they were recovered in a bucket. It was noted that a filmformed around each drop as it passed from the top of the polymersolution to the bottom of the vessel. The polymer coated drops wereremoved from the vessel by raising the bucket and decanting the carbontetrachloride from the capsules. The recovered capsules were then washedwith additional carbon tetrachloride and then with petroleum ether. Thedried capsules were less tacky than those recovered from theencapsulation zone prior to washing. One of the capsules was melted on ahot plate and it was noted that it produced red fumes which indicatedthe presence of active red fuming nitric acid. The remaining capsuleswere admixed with aluminum powder in a 1:1 weight ratio and it was notedthat no reaction took place at room temperature.

Example 2 Example 1 is repeated except that a liquid mixture consistingof wt. percent RFNA and 95 wt. percent tetranitromethane is used inplace of RFNA.

Example 3 Example 1 was repeated except that a polystyrene (100,000M.W.) solution was used in place of the styrene-isobutylene copolymer.

Example 4 Example 1 was repeated except that styrene-butadiene copolymer(SBR) having a Staudinger molecular weight of 92,000 was used in placeof the styrene-isobutylene copolymer.

Resort may be had to various modifications and variations of the presentinvention without departing from the spirit of the discovery or thescope of the appended claims.

What is claimed is:

1. A solid-liquid rocket propellant system which comprises a mixture ofnitric acid as liquid oxidizing agent in a plurality of nitratedpolymeric hydrocarbon capsules A to A in diameter and a solidmetal-containing fuel selected from the group consisting of aluminumpowder, boron powder, magnesium powder and hydrides of said metals, theweight ratio of liquid oxidizing agent in the capsules to the solid fuelmixed with the capsules being about 1:1 to 1:2.

2. A solid-liquid rocket propellant system as set forth in claim 1, inwhich liquid oxidizing agent in said capsules contains 1 to 20 wt.percent of nitric acid and to 99 wt. percent of C to C nitroparafiin.

3. A solid-liquid rocket propellant system as set forth in claim '1, inwhich the nitric acid in said capsules is red fuming nitric acid and thenitrated polymeric hydrocarbon is nitrated styrene-isobutylenecopolymer.

4. A solid-liquid rocket propellant system which comprises smallsemi-rigid capsules of a nitrated hydrocarbon rubber containingencapsulated nitric acid as liquid oxidizing agent, said capsules beingabout & to about A in diameter and being mixed in a weight ratio ofabout 1:1 to 1:2 of a solid fuel selected from the group consisting ofaluminum powder, boron powder, magnesium powder and hydrides ofaluminum, boron and magnesium.

5. The method of preparing a solid-liquid propellant system whichcomprises contacting discrete drops of a liquid containing nitric acidwith a solution of a rubbery polymeric hydrocarbon in an encapsulatingzone, reacting the discrete drops of liquid with the polymer hydrocarbonin solution to form a nitrated film of the polymer surrounding the dropof liquid containing the nitric acid, controlling the size of saiddrops, contact time and viscosity of the polymer solution to obtaincapsules having a diameter of A to A, withdrawing the resulting capsulesfrom the encapsulating zone, mixing the capsules containing nitric acidas liquid oxidizing agent with a powdered metal fuel selected from theclass consisting of aluminum, boron, magnesium and hydrides thereof in aweight ratio of from 1 part of encapsulated liquid oxidizing agent to1-2 parts of the powdered fuel.

References Cited UNITED STATES PATENTS 2,541,165 2/1951 Kulp 18482,702,924 3/1955 Plourde 1848 2,712,497 7/1955 Fox et al. 52-0.52,783,138 2/ 1957 Parsons 52--0.5 2,802,332 8/1957 Orsino 60-3562,960,935 11/1960 Colpitts l0270 3,143,446 8/1964 Berman 1492 OTHERREFERENCES Zaehringer, Solid Propellant Rockets Second Stage, AmericanRocket Co., Box 1112, Wyandotte, Mich. (1958) pp. 229 to 231.

BENJAMIN R. PADGETT, Primary Examiner.

1. A SOLID-LIQUID ROCKET PROPELLENT SYSTEM WHICH COMPRISES A MIXTURE OFNITRIC ACID AS LIQUID OXIDIZING AGENT IN A PLURALITY OF NITRATEDPOLYMERIC HYDROCARBON CAPSULES 1/64" TO 1/4" IN DIAMETER AND A SOLIDMETAL-CONTAINING FUEL SELECTED FROM THE GROUP CONSISTING OF ALUMINUMPOWDER, BORON POWDER, MAGNESIUM POWDER AND HYDRIDES OF SAID METALS, THEWEIGHT RATIO OF LIQUID OXIDIZING AGENT IN THE CAPSULES TO THE SOLID FUELMIXED WITH THE CAPSULES BEING ABOUT 1:1 TO 1:2.